Posted
by
ScuttleMonkey
on Monday March 24, 2008 @05:33PM
from the bouncing-off-walls dept.

esocid writes to share that University of Maryland physicists have demonstrated that the material of the future may be graphene rather than silicon. Electricity conduction through graphene is about 100 times greater than that of silicon and could offer many improvements to things like computer chips and biochemical sensors. "Graphene, a single-atom-thick sheet of graphite, is a new material which combines aspects of semiconductors and metals. [...] A team of researchers led by physics professor Michael S. Fuhrer of the university's Center for Nanophysics and Advanced Materials, and the Maryland NanoCenter said the findings are the first measurement of the effect of thermal vibrations on the conduction of electrons in graphene, and show that thermal vibrations have an extraordinarily small effect on the electrons in graphene."

...refers to electron mobility [wikipedia.org], a concept I hadn't previously encountered. But it's easy enough to understand: if I apply a unit electric field to a material, how fast does it make the electrons drift? This is the mobility.

Apparently graphene (also new to me... a single-atom layer of carbon) is exciting because it has much higher electron mobility than silicon. Which leads to faster switching times, although they don't explain that part.

All this seems to be theoretical at the moment, due to insufficiently pure graphene. Still, 100th the switching delay is not a bad target to be aiming at... 100Ghz processing!

...refers to electron mobility [wikipedia.org], a concept I hadn't previously encountered. But it's easy enough to understand: if I apply a unit electric field to a material, how fast does it make the electrons drift? This is the mobility.

Apparently graphene (also new to me... a single-atom layer of carbon) is exciting because it has much higher electron mobility than silicon. Which leads to faster switching times, although they don't explain that part.

All this seems to be theoretical at the moment, due to insufficiently pure graphene. Still, 100th the switching delay is not a bad target to be aiming at... 100Ghz processing!

Thanks, now I don't have to RTFA. I was wondering why pure conductivity improvements are good for gates. Semiconductors are used for a reason.:-)

Thanks, now I don't have to RTFA. I was wondering why pure conductivity improvements are good for gates. Semiconductors are used for a reason.:-)

The increased mobility has little to do with gates. In fact, you want gates (in MOSFETs [wikpedia.org]) to be as resistive as possible, but still not attenuate the electric field that results from the gate voltage, hence the use of Halfnium dioixde instead of silicon dioxide (you can make it thicker, (and thus more resistive) while still having a strong enough field.)

Mobility results from the equation v=(mu)E, where mu is the mobility and v is the velocity of an charge carrier (electron or hole) The reason we use semiconductors is that we can easily control the number of electrons or holes. But by increasing the speed of electrons, we can allow them to switch faster since they will be able to cross the channel more quickly. That's why smaller transistors can switch more quickly, the channel length is shorter so it takes less time for carriers to traverse them.

I'm not sure why it's considered so amazing to discover that graphene has a good electron mobility. Since, the entire structure consists of delocalized pi orbitals, you would expect electrons to easily travel through graphene. I'm not sure how graphene would be doped either. I suppose you could use boron and phosphorous like in silicon, but it remains to see if they will still bond appropriately. Ah well, there's a reason, they're professors and I'm a student.

Science writing is frustrating if you know anything at all about what's being discussed. It's dumbed down to the point that you feel less informed for having read it. They invariably leave out the "trivial" little detail that makes it all make sense. They might as well just write, "Something new and nifty and important has been discovered! But it's too complicated to explain it to you, so we'll spare you the boring, complicated details."

As I understand it, a "hole" is just the absence of an electron, which leads to a net positive charge for a particular atom. Kind of like a positive ion, but I think use of the term "ion" is limited to liquid solutions/gases/plasmas.An electron can move and fill a hole, but leaves another hole behind in the location it just departed. So a "hole" moving in one direction is entirely equivalent to an electron moving in the opposite direction, is it not?

Representing charge as "holes" is useful for current said to be flowing from a higher voltage (lacking electrons) to a lower voltage. The electrons are actually going from where they are in excess (giving a more negative charge) to where they are lacking. Therefore, the "holes" and electrons are trading places. It's like heat being dissipated, and saying "cold" is moving in.

The way you describe the motion of electrons and holes as being equivalent but in opposite directions is a very good way to look at i

Electrons are normally attached to an atom. However, at temperatures above absolute zero, some electrons from an atom can leave the atom. When an electron leaves an atom, it leaves behind a hole; because a hole can be thought of as an absence of an electron, it has the same magnitude charge, but opposite sign, and a hole is also mobile, just like a free electron. Just as electrons can spontaneously leave an atom, it can recombine with a hole, and they both "annihilate" each other; for any given temperatu

As I understand it, a "hole" is just the absence of an electron, which leads to a net positive charge for a particular atom. Kind of like a positive ion, but I think use of the term "ion" is limited to liquid solutions/gases/plasmas.

First of all, holes are not the result of net positive charges in an atom. In silicon, you can create holes by replacing a silicon atom in the crystal lattice with a boron atom. Silicon has 4 valence electrons, but boron only has 3. Therefore, the silicon atom that is replaced by the boron could form 4 bonds with neighboring silicon atoms. The boron can only form 3. The remaining empty space is what we call a hole. Note that nothing is electrically charged.

An electron can move and fill a hole, but leaves another hole behind in the location it just departed. So a "hole" moving in one direction is entirely equivalent to an electron moving in the opposite direction, is it not?

If so, why does this term have any usefulness, if, instead of saying "the hole moved from point A to point B" you could just as easily say "the electron moved from point B to point A"?

Your characterization of n-type silicon is wrong. There are not a number of unfilled holes; instead, there is an extra electron that is not involved in any covalent bonds, and is free to move about.
A better model, still in ASCII would look like this:.......:..........
where the extra dot in the colon shows the extra electron.

Additionally, that extra electron is not forced into the conduction band; some small minority will still be associated with an atom at room temperature. Additionally, the percen

Silicon is the material of choice because of its good oxide and because engineers have, what, 60 years of experience with it now? Limits to scaling down silicon based chips come from silicon oxide not being a good enough dielectric (insulator) and from very small 'off' transistors letting through too much leakage current. More conductive materials aren't particularly helpful in that regard.

Polycrystal silicon is used for transistor gates and routing signals over very short distances, maybe they mean to r

Does "higher electron mobility" necessarily mean more conductive in the "off" state? I thought it just meant faster switching.

I was just pointing out that it doesn't address the significant problems in current devices. I don't think it will significantly affect either if it were used as a replacement semiconductor (if it even could be used that way). When materials like carbon nanotubes are made into 'transistors' the device changes significantly and generally works on different principles that current devices, so what holds for silicon transistors goes out the window.

Graphene has been studied for a few years now, even longer if you count it as rolled into a nanotube.

What took awhile (and was solved with a fairly low-tech solution : scotch tape) was how to make a single layer of graphene to measure, whereas graphite usually rolled off into multi-layer pieces.

Graphene is interesting for a number of reasons. Primarily is it's Minkowski lightcone-like density of states. The Fermi level lies right at the cone vertex, which makes this material a "zero-bandgap insulator", which brings about a huge number of interesting properties in itself.

Anyway, graphene has been hugely popular in condensed matter physics for a few years now, and people have studied the phonon spectra, I remember going to a seminar about the modes of graphene in a carbon nanotube a few years ago.

However, don't get your hopes up for mass-produced graphene tech anytime soon. While people will probably demonstrate small-scale single-electron transistors or other interesting graphene devices (if they haven't already), the ability to deposit and pattern graphene is still very crude, and it's hard to do anything other than one-off devices at this point.

It's also very hard to "solder" interconnects on a single layer sheet. Alnd, due to the 2 dimensional nature of the graphene sheet you can't easily take advantage of modern multilayer silicone processing. Making a true device from this will be challenging.

I met one of the students from the GT group recently and he mentioned the scotch tape solution and said he said his lab were investigating how to manufacture the material practically. For all it's promise, I got the impression that two or three major breakthroughs were needed to make it viable. It's definitely a few years away. (I me

I remember reading about how physicists are running into the limitations of "C" (speed of light) with regards to signal propagation across the CPU die. Even though something measuring 143 mm^2 is small, at speeds of 100GHz (or was that 1Thz), I doubt your processing can remain symmetrical. If that's true, such fast CPUs will need to be engineered for asymmetrical processing instead.

On the other side of the coin, the design for an original Pentium had around 5 million transistors. Modern processors have more like 300 million. What's changed? Well, dual-core, and 64-bit, sure. But a lot of those extra transistors are to create extra pipelines or additional specialized instructions or even specialized pipelines that only run specialized instructions to compensate for the fact that the clock speeds just won't ramp up as quickly as designers want. Perhaps if we were able to start cranking up the clock speeds again, it would be possible to start streamlining those pipelines and instruction sets into something more manageable for keeping your signals properly synchronized.

Basically the CPU manufacturers now have technology to put lots of transistors on a die (up to billions even), but they haven't figured out how to use them all to make things go faster - it's hard to think of new tricks to make a CPU _intelligently_ faster (e.g. process more instructions per cycle).Cache is the easy way out - but it doesn't necessarily give you much better performance beyond a certain size (for typical workloads anyway).

So what do you do if you have so much transistors left, that using all

Besides, rendering the holotextures required to accurately represent the shape and movement of disembodied hands is no small task. In fact, it's so difficult that it will not be supported until Windows 17 (aka 'Fettershorn' [wikipedia.org]) is released. Never mind the fact that the requirements for that edition are so steep that it'd requi... hold on a sec, someone's at the door...

Unfortunately the article doesn't mention a lot of other important characteristics of semiconductors. From what the article said, they basically have a new material for traces on PCBs, although at just one atom thick those would be easily damaged. For a semiconductor they have to be able to dope it. AFAIK carbon can be doped P but not N, so unless they can figure that out as well, it won't be any better than what we have now.

Still, 100th the switching delay is not a bad target to be aiming at... 100Ghz processing!

Wow, that is some serious speed. Imagine if you tried to get an old Prescott P4 to run at not 3, not 4, but 100Ghz...You could run Far Cry, Prey and Crysis at the same time. Of course the heat would probably turn the planet to a gas, but there's a downside to everything I guess...

I'm waiting for my one MegaCore processor with 1,048,576 cores, while mocking the market-war with MegiCore processors who only have 1,000,000 cores, but perform better at rendering realistic 3D models of females.

Aluminum is far, far more prone to oxidation than crystalline carbon. That's all graphene is anyway, a 2-D crystal of carbon. Aluminum oxidizes easily at room temperature. Carbon does not. It takes a lot of thermal energy (think burning) to convince carbon to let go of itself and start grabbing oxygen instead.

Yes. the linked article shows photomicrographs of quantum dots made on graphene surface that are set up via doping and can act as gates. I'm going to guess that perhaps a resistive base will be used, photolithographed, and via some magic process the graphene "wires" will be deposited onto the base into the channels or, perhaps pressed onto the ridges, before being doped further.

If electron mobility was important silicon would have been replaced by Galium Arsenide years if not decades ago. GaAs can pass all of the first 3 requirements suggested by the parent - but not in a scalable way. For example you can get a good quality insulator on it, but its just bloody hard to do.

"how that thermal vibrations have an extraordinarily small effect on the electrons in graphene" does this mean that graphene transitors will have HFE as a stable paremeter?!? that would be seriously awsome!

Graphene wasn't even [b]fabricated[/b] for the first time until 2004 by the so-called "Manchester group". Carbon nanotubes were formally identified in 1991 and intentionally created shortly thereafter and we've done what exactly with them? As far as I know, companies like Nantero [nantero.com], which uses carbon nanotubes as a basis for nonvolatile memory, are few and far between. I'm active in the field, and I can just say it's going to be a year or two until we even see transistor demos much less arrays of memory or

Chemists are going to be pissed to hear this! New periodic tables, new reactions, new names for all sorts of all sorts of chemicals. And where is the Silicon going to go? Also -- what are the geologists going to think? Over 25% of the earth's crust is made of the stuff!

Graphene is certainly a lot like carbon nanotubes, but is much easier to work with. Where you have to hope to get a semiconducting crystal structure in a nanotube (or make crappy transistors based on defects), you can pattern graphene to make a transistor. Which directions you cut the 2D sheet determine whether it is metallic or semiconducting. There are some problems with this, and practically speaking any small channel (10 nm, I think) of graphene is semiconducting. Fuhrer has shown (along with other people) that graphene can make pretty good transistors (very fast switching, thermally stable and I'm sure I'm missing some stuff).

It can be doped. This is another thing Fuhrer has done (as well as other people... but this is his article we're talking about). You don't want to insert something into the crystal structure (that ruins it), but you can layer the top of it with potassium ions (about 1 per 1000 carbons), which dopes it just fine. This isn't a bulk semiconductor though, and the addition of charged impurities (dopants) decreases device performance (in bulk, it's a metal). You can very easily electrostatically gate graphene in any direction you want; transistors and PN junctions are easy to make this way.

It is not hard to make graphene. The "scotch tape" method from Manchester is widely used, but there are a number of other ways to do it which may be commercially viable: oxidizing graphite, ultrasounding graphite with special polymers (Dai's method), growing it from SiC wafers. Of course, none of these really work yet, and may never be economical.

Graphene is stable in air (almost all devices are measured in air at some point), and liquids. It's not going to spontaneously dissolve on you just because it's only 1 atomic layer thick. It's actually very robust.

It can be used with silicon processing techniques. People are using SiO2, HfO2 and all the usual silicon processing with it.

Big companies are looking at this material. IBM has already reported results on their work at physics conferences, I'm fairly sure that the more secretive companies (Intel) are also working with graphene... just like they worked with nanotubes.

A friend of mine works for Prof Andre Geim in the Mesoscopic Physics Group [man.ac.uk] at University Manchester and was one of the people to first prepare graphene crystals. They have a spin off that is selling graphene flakes to some interested industry altho the demand is not huge at the moment. If you want to play with graphene flakes of your own you can check them out here [grapheneindustries.com].

It is interesting stuff - I saw Prof Geim speak about it and it seems to me one of these areas where quantum theory and experiment intersect,

I didn't read the article (this is/. after all), but if it's made of graphite spread really thin (one atom thick), isn't it still just graphite? Just really thin? It sounds more like they discovered special properties in application of the material, as opposed to what it does when sitting in my pencil.

They are MANY materials with superior electronic characteristics to silicon. the original transistor industry used gallium, arsenic, and indium. However nothing comes close to a billion devices on chip for $100.